I found this in my lounge today. This room was cleaned less than 48hours ago, with no sign of this growth.
We sweeped and mopped, using the same floor cleaner we always do.
The two images below is of the same corner, but with the growth lifted in one image.
The consistency is like a fungus or mushroom and can probably be removed from the wall as one piece without much effort.
One of the walls close by do suffer from damp on occasion, but we have been in this house for over two years and have not had anything like this appear.